The present invention relates to a novel ferrimagnetic material. More particularly, it relates to a lead-substituted rare-earth iron garnet having the compositions of a rare-earth iron garnet where rare-earth element is substituted by lead. The term "rare-earth iron garnet" used herein means a synthetic iron garnet of yttrium and rare-earth elements.
Discovery of synthetic iron garnet, especially yttrium iron garnet brings about new interest in the field of ferrimagnetic materials and it is extensively used as an isolator in microwave transmission system. Recently, the iron garnet is widely used as an optical modulator, a magnetic flux sensor, a magneto-optical memory and a bubble domain device, utilizing its Faraday effect.
In case of the conventional synthetic iron garnet, e.g., simple rare-earth iron garnet, preparation temperature of single garnet phase is about 1400.degree. C which is considerably high and figure of merit representing its specific Faraday rotation is lower by about 0.2. Furthermore, as disclosed in Japanese Pat. No. 5344/68, preparation temperature of single garnet phase of (RBi)Fe.sub.5 O.sub.12 which has the compositions of a rare-earth iron garnet in which a part of rare-earth element is replaced by Bi is about 1,000.degree. C. This (R.sub.3.sub.-x Bi.sub.x)Fe.sub.5 O.sub.12 has a lower preparation temperature of single garnet phase than the simple rare-earth garnet, but the former preparation temperature is still high.